1. Field of the invention
The present invention relates to an image reading method and apparatus therefor wherein CCDs and similar devices are utilized, especially when making adjustments to compensate for read-out position errors.
2. Description oft he Prior Art
FIG. 1 shows the principle employed in a prior art apparatus for reading images where a CCD line sensor is utilized. An original image sheet 2 is set on a table 4. A light source, not represented here, is provided above or below the table 4. Beams from the light source are reflected by or pass through the original image 2, then pass through a lens 6 and are focused on a line sensor 8. As the line sensor 8 is comprised of a series of sensors arranged in a line, each individual sensor outputs a signal level corresponding to the degree of density for that portion of the line being scanned from the original image 2.
The table 4 is moved in a secondary direction S, for the subsequent scanning of adjacent lines, by rotating a motor 10. This movement is repeated until all adjacent lines of the original image 2 have been scanned. Using this technique, signal levels corresponding to each point on the original image 2 are obtained.
When a color image is desired for printing and reproduction, the reflected light is separated into an R component, G component and B component, by the use of three line sensors, one for each of the color components. Using the technique for scanning the original image 2, color signals corresponding to each component for an entire image may be obtained. Alternatively, three different colored light sources may be used sequentially with one line sensor to form the R.G.B. signals representing the original image 2.
If there is optical distortion in the primary scanning direction P of an optical system, for example in lens 6, the line sensor 8 will output signals that include the distortion as part of the information contained in the signal. Optical systems without distortion are difficult to build and expensive, especially if the line sensor is used to receive images that are magnified by the optical portions of the scanner. For these reasons, it is difficult to obtain undistorted information in scanning signals. Furthermore, when color image signals are obtained, there will often be an error of the positional information between the signals for the different color components corresponding to the different aspects of aberration introduced by transverse chromatic aberration of the optical system. As the complexity of the optical system increases by requiring color information and other such demands on the optical characteristics of the system, the costs and set-up time increase dramatically while an increase in performance becomes more and more difficult to achieve.
If three line sensors 8 are used to obtain color images, the respective individual sensors in each line sensor 8 must be aligned to correspond to the same point in the original image 2. That is, a positional arrangement must be achieved such that light from a single point on an original image will strike the same sensor respectively in each of the separate line sensors. This positional arrangement is both elaborate and difficult to achieve as well as costly to manufacture.